The present disclosure relates to an electrical component having overlapping electrodes, for example a capacitor or a bulk oscillator, in particular a thin-film resonator or FBAR (Thin Film Bulk Acoustic Wave Resonator).
The electrodes, situated one over the other, of a thin-film resonator are produced in metal layers, for example in an exposure method using exposure masks. The areas of the electrodes that lie exactly one over the other define through their area of overlap an active surface, that is, a surface that is of decisive importance for the electrical characteristics of the component. Errors in adjustment in the placement of the mask result in manufacturing-related scatter losses of the active surface, and, in particular in filter applications, to a resulting worsening of the electrical characteristics.
Unintentional overlappings of the electrodes of a thin-film resonator result in parasitic resonators through the piezoelectric layer situated therebetween, whose admittance curves (frequency response curve of the admittance) are shifted against the admittance curve of the original resonator. The parasitic resonators are connected parallel to the original resonator (main resonator) and thus have an adverse effect on its bandwidth and quality. For example, a parasitic resonator whose surface is only 0.002% of the surface of the main resonator, and whose parasitic resonance is close to the anti-resonance of the main resonator, reduces the quality of the main resonator in the area of the anti-resonance by approximately 50%.
For example, for resonator and filter applications in the frequency range between 500 MHz and 5 GHz, a resonator surface of 200 μm×200 μm is typically used. An adjustment error of the masks, resulting in a mutual shifting of the electrodes, whose side length is 200 μm, by only 0.4 μm results in a reduction of the resonator quality to one-half of the ideal value.
In a bandpass filter, a reduced quality of the resonators results in a low edge steepness in the passband of the transmission function. The edge steepness is thereby decisive for the fulfilling of the specifications in filter applications, in which the passband and the rejection bands or blocking-state regions are very close to one another in terms of frequency. In addition, the reduction of the resonator quality also results in an increased insertion loss and a reduced bandwidth.
Up to now, mask adjustment errors were removed for example by the later removal of errored regions (see the reference U.S. Pat. No. 5,894,647). However, this solution has the disadvantage that an additional method step is required, which increases the processing time.
It is also possible to design one of the electrodes to be larger, taking into account the tolerance shifts. However, in this case the alteration of the overlapping surface, with the shift of the corresponding electrical supply line or conductor cannot be compensated. Moreover, parasitic capacitances of the component are thereby increased.
Another possibility for avoiding the tolerance errors in thin-film resonators is to use high-precision mask positioning in the lithography. For example, in contact lithography the precision of positioning is approximately ±1.0 μm, in thin-film stepper lithography it is approximately ±0.1 μm, and with the most modern step-scan systems it is approximately ±0.01 μm. However, the use of such devices results in a considerable increase in the processing costs, and can, for example in DRAM manufacturing, account for up to 50% of the overall process costs.